The precise temperatures and times that are required when firing dental porcelains will vary from manufacturer to manufacturer and furnace to furnace.

After each individual firing, the temperature may be slightly reduced (5-10 deg C) so each firing will not affect the previous firing. However, it important to know that this is not necessarily always the case, as in some modern porcelains, each consecutive dentine cycle after the first retains the same temperature. Below are three main considerations for the firing of dental porcelains, and how they are managed by the firing programme.

All porcelains require slow, controlled heating and cooling to prevent thermal shock (causing cracking). This will vary, but generally, a 6 minute preheat is required when firing margin or dentine cycles. This is controlled by the speed of the muffle closing, combined with the rate of temperature increase within the muffle during the cycle.
In order to provide a slow cool, most furnaces reduce power to the heating element and begin a gradual opening of the muffle. Preventing thermal shock is particularly important when firing bonded porcelains.
All modern dental porcelains are designed to be fired in a partial vacuum. Exactly when the vacuum activates in the cycle will vary, depending on the stage, the porcelain type and the furnace itself. The same is said for when the partial vacuum is released.

The purpose of creating a vacuum during the firing process is to reduce the risk of porosity within the porcelain, by removing trapped gases from the material as the particles sinter. This increases the strength of the porcelain, and also increases the aesthetic properties. The porosity volume has a detrimental effect on the aesthetics of the material by increasing opacity.

The final grain size is determined by conditions present during the sintering phase, such as firing duration and firing temperature. Higher temperatures and longer cycles lead to larger grain size. The strength and opacity of aluminous porcelain is related to the grain size of the alumina crystals; the grain size of the fused alumina crystals is selected to provide optimum strength without producing excessive opacity. The finer the grain size the greater the strength but this is accompanied by increased opacity.

Using too low a temperature will prevent the alumina particles from full fusing, weakening the porcelain structure.

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Explain, with the use of diagrams,how to sprue porcelain crowns forpressable ceramic materials